Multi-echo gradient echo (mGRE) sequences have been widely adapted in clinical and scientific practice for different purposes due to their capability of generating multi-contrast images and extracting multi-parametric maps. Using the magnitude mGRE data, R2* relaxivity (R2*=1/T2*) mapping techniques (1) have been used to quantify blood oxygenation level dependent functional MRI, detect and track of super-magnetic iron oxides, visualize abnormalities of the articular knee, assess iron content in brain, liver and heart. However, a few newer techniques have been developed using the commonly discarded phase data. Among these, susceptibility weighted imaging (SWI) (2) uses the phase information to enhance the susceptibility contrast of the magnitude image to visualize veins, microbleeds, hemorrhage, clot, etc; local frequency shift (LFS) mapping techniques (3) characterize anatomical structures based on phase-contrast features; relying on the LFS maps, quantitative susceptibility mapping (QSM) techniques (4) have also been explored to quantify susceptibility (χ), which is an intrinsic property of materials (e.g., iron, calcium, biological tissue). Also, a few mGRE-based Dixon MRI techniques (5-7) have been developed to jointly estimate B0 inhomogeneity, proton density fat-fraction (FF), R2* and χ, using the complex mGRE data.
The mGRE sequences can be prescribed to cover a large volume in clinically acceptable scan times when the acquisition is conducted using multi-channel RF coils with a large number coil-elements and employing parallel imaging techniques. Compared to 2D multi-slice acquisitions, 3D mGRE acquisition strategies can also generate high-resolution (≤1 mm) data sets with higher SNR (8) and have been proven to be useful for many applications, such as stroke, oncology, multiple sclerosis, etc. However, the performance of the R2*, SWI, LFS and QSM techniques for imaging large volumes might be downgraded by many confounding factors, particularly, the presence of macroscopic B0 inhomogeneity and fat content (9). A popular approach to avoid the effects of fat on the measured R2* is to acquire only “in-phase” echoes, i.e., the prescribed echo times (TEs) lead the phase differences caused by chemical shift between fat and water to be equal to a multiple of 2π. However, the “in-phase” approach is based on a single-peak fat model, despite the fact that the fat spectrum has many peaks. If an accurate B0 map is known, the effect of B0 on the measured R2* could be compensated. Because macroscopic B0 inhomogeneity is also the dominant source of the measured phase signal, an accurate B0 map is crucial for the accurate background-phase removal performed for all phase-sensitive techniques (Dixon MRI, SWI, LFS and QSM). Furthermore, an accurate B0 map enables elimination of the phase component caused by chemical shift when fat is present.
In most of the reported work (10), a set of TEs optimized for Dixon Mill, i.e., short first TE and small echo spacing (ΔTE), was used for data acquisition. U.S. Patent Application 2014/0142417 A1 discloses a method of using the separated water and fat images, as well as B0 inhomogeneity map to estimate the QSM values from Dixon Mill data. U.S. Patent Application 2015/0002148 A1 discloses a method of joint estimating fat-water fraction and the QSM values by iteratively refining fat chemical shift.
However, a set of long echo times (TE) with large ΔTE may be needed to match the tissue T2* and optimize susceptibility effects for some cases, e.g., brain imaging using R2*, SWI, LFS and QSM. While the concept of acquiring mGRE images with two echo trains has been considered previously (11), no process has been developed which fully optimizes the acquisition parameters, and corrects for the effect of B0 on the phase-sensitive images.